The Obama administration invokes the Voting Rights Act in blocking the measure, saying it could deny the right to vote for tens of thousands of blacks and other minorities.

Reporting from Washington — The Obama administration's civil rights office is stepping up its fight with the Southern states over voting rights, announcing it will block a new South Carolina law that would require voters to show a government-issued photo identification before casting a ballot.

The Justice Department invoked the Voting Rights Act on Friday and said the new photo-identification rule could deny the right to vote for tens of thousands of blacks and other minorities.

"According to the state's statistics, there are 81,938 minority citizens who are already registered to vote and who lack DMV-issued identification," Thomas E. Perez, the chief of the department's civil rights division, said in a letter to South Carolina officials. He referred to a driver's license issued by the state Department of Motor Vehicles, the most common form of photo identification.

Under current law, a South Carolina resident who is registered to vote can cast a ballot if he or she has a voter registration card and a signature on the polling list, Perez said.

South Carolina was one of many states to enact new laws earlier this year that tighten the rules for voting. Last week, Atty. Gen. Eric H. Holder Jr. spoke out against these laws, describing them in the words of Rep. John Lewis (D-Ga.) as "a deliberate and systematic attempt" to prevent millions of elderly, low-income and minority Americans from voting.

Under Section 5 of the Voting Rights Act, Southern states with a history of racial discrimination must seek advance approval from the Justice Department or a federal court in Washington before adopting new election laws.

South Carolina Gov. Nikki Haley vowed to get the decision overturned.

"The president and his bullish administration are fighting us every step of the way. It is outrageous, and we plan to look at every possible option to get this terrible, clearly political decision overturned so we can protect the integrity of our electoral process and our 10th Amendment rights," Haley, a Republican, said in a statement.

The Obama administration is also locked in a Supreme Court fight with Texas and its Republican Legislature over the proposed remapping of its new election districts.

Still under review are new election laws from several other Southern states, including Texas and Florida.

Photo-identification laws have divided Republicans and Democrats across the country. Republicans say this requirement will help prevent fraudulent votes. Democrats say there is virtually no evidence that impostors show up claiming to be registered voters.

Three years ago, the Supreme Court upheld a photo-ID law in Indiana, noting the state promised to provide free photos to all who were eligible to vote.

Southern lawmakers assumed that decision would shield their new laws from legal attack. But the Voting Rights Act empowers the Justice Department to stand in the way if a new election law is likely to have a discriminatory effect on blacks and Latinos.

South Carolina is free to seek a reversal from a three-judge federal court.

Victoria Middleton, executive director of the ACLU in South Carolina, called the new law "misguided" and a threat to "one of our most cherished and basic rights. We are pleased to see it stopped in its tracks."